1. Field of the Invention
The present invention relates to an electronic article surveillance system and a non-deactivatable marker for use therein; and more particularly, to a process for fabricating a magnetomechanically resonant, non-deactivtable marker with improved control of the resonant frequency of the marker that enhances the sensitivity and reliability of the article surveillance system.
2. Description of the Prior Art
Attempts to protect articles of merchandise and the like against theft from retail stores have resulted in numerous technical arrangements, often termed electronic article surveillance (EAS). Many of the forms of protection employ a tag or marker secured to articles for which protection is sought. The marker responds to an electromagnetic interrogation signal from transmitting apparatus situated proximate either an exit door of the premises to be protected, or an aisleway adjacent to the cashier or checkout station. A nearby receiving apparatus receives a signal produced by the marker in response to the interrogation signal. The presence of the response signal indicates that the marker has not been removed or deactivated by the cashier, and that the article bearing it may not have been paid for or properly checked out.
One common type of EAS system typically known as a harmonic (or electromagnetic) system relies on a marker comprising a first elongated element of high magnetic permeability ferromagnetic material, which is optionally disposed adjacent to at least a second element of ferromagnetic material having higher coercivity than the first element. When subjected to a low-amplitude electromagnetic field having an interrogation frequency, the marker causes harmonics of the interrogation frequency to be developed in a receiving coil. The detection of such harmonics indicates the presence of the marker. A marker having the second element may be deactivated by changing the state of magnetization of the second element, typically by exposing it to a dc magnetic field strong enough to appreciably saturate the second element. Depending upon the design of the marker and detection system, either the amplitude of the harmonics chosen for detection is significantly reduced, or the amplitude of the even numbered harmonics is significantly changed. Either of these changes can be readily detected in the receiving coil. In practice, harmonic EAS systems encounter a number of problems. A principal difficulty stems from the superposition of the harmonic signal and the far more intense signal at the fundamental interrogation frequency. The detection electronics must be responsive to the relatively weak harmonic signal and discriminate it from the carrier signal and other ambient electronic noise. Harmonic systems are also known to be vulnerable to false alarms arising from massive ferrous objects (such as shopping carts) also present in a typical retail environment.
Another type of EAS marker and system (known as magnetomechanical or magnetoacoustic) is disclosed by U.S. Pat. Nos. 4,510,489 and 4,510,490 (“the '489 and '490 patents”), both to Anderson et al., which are both incorporated herein in the entirety by reference thereto. The marker comprises an elongated, ductile strip of magnetostrictive ferromagnetic material adapted to be magnetically biased and thereby armed to resonate mechanically at a frequency within the frequency band of an incident magnetic field. A hard ferromagnetic element, disposed adjacent to the strip of magnetostrictive material, is adapted, upon being magnetized, to arm the strip to resonate at that frequency. The resonance condition is established by the equation:fr=(½L)(E/δ)1/2  (1)wherein fr is the resonant frequency for an elongated ribbon sample having length L, and E and δ are the Young's modulus and mass density of the ribbon, respectively.
The resonance causes the marker to respond to an ac electromagnetic field by changes in its mechanical and magnetic properties, notably including changes in its effective magnetic permeability. In the presence of a biasing dc magnetic field the effective magnetic permeability of the marker for excitation by an applied ac electromagnetic field is strongly dependent on frequency. That is to say, the effective permeability of the marker is substantially different for excitation by an ac field having a frequency approximately equal to either the resonant or anti-resonant frequency than for excitation at other frequencies. Exposing the resonant element to an external ac field urges it to vibration, with a coupling that may be characterized by the marker's magnetomechanical coupling factor, k, greater than 0, given by the formula:k=[1−(fr/fa)2]1/2,  (2)wherein fr and fa are the resonant and anti-resonant frequencies of the magnetostrictive element, respectively. A detecting means detects the change in coupling between the interrogating and receiving coils at the resonant and/or anti-resonant frequency, and distinguishes it from changes in coupling at other than those frequencies. The coupling is especially strong for excitation at the natural resonant frequency. It is further known, e.g. from U.S. Pat. No. 5,495,230 to Lian, that the resonant frequency depends strongly on the magnitude of the biasing field imposed on the resonant element as a consequence of the bias-field dependence of Young's modulus E in the foregoing resonance equation.
A marker of the type disclosed by the '489 patent is depicted generally at 11 by FIG. 1. Marker 12 comprises a strip 14 disposed adjacent to a ferromagnetic element 16, such as a biasing magnet capable of applying a dc field to strip 14. The composite assembly is then placed within the hollow recess 17 of a rigid container 18 composed of polymeric material such as polyethylene or the like, to protect the assembly against mechanical damping. The biasing magnet 16 is typically a flat strip of magnetic material such as SAE 1095 steel, Vicalloy, Remalloy or Arnokrome. Magnetomechanical EAS systems in which it is desirable to deactivate the marker in the field usually employ semi-hard magnetic materials for the bias element.
The '489 patent also discloses a pulsed EAS system in which a transmitter drives a transmitting antenna, such as a coil, that produces a pulsed electromagnetic field having an interrogation frequency. If present within the antenna field, an active marker having a resonance frequency equal to the interrogation frequency is driven into magnetomechanical resonance. During the interval between transmitted pulses, the excited marker continues to vibrate mechanically at its resonant frequency, thereby producing a magnetic field oscillating at the resonant frequency. The amplitude of the mechanical vibration and the resulting magnetic field decrease exponentially with time. This damped resonance thereby provides the marker with one form of characteristic signal identity.
A similar EAS marker disclosed by the '490 patent comprises multiple strips disposed in a side-by-side fashion. The strips have different resonant frequencies, permitting the marker to be coded by selecting particular frequencies. The coding is detected by ascertaining the multiple frequencies at which the '490 tag exhibits resonance.
However, known magnetomechanically resonant markers comprising magnetostrictive material and systems employing such markers, including those of the types disclosed by the '489 and '490 patents, have a number of characteristics that render them undesirable for certain applications. The markers are relatively large in size, in both their length and width directions. As a result, they are too large to be accommodated on some items of merchandise, including many for which protection is highly desirable because of their high value. A large marker is also relatively conspicuous when affixed externally to a merchandise item. Attempts to reduce the size of the marker encounter certain obstacles. In general, reducing the volume of the resonating magnetic element proportionally reduces the detectable signal from the marker and the size of the interrogation zone within which the marker is responsive, hindering reliable detection. For example, in a retail environment, it is a practical necessity that reliable detection be possible over the full aisle width at the store's exit.
Another form of magnetoacoustic EAS marker is provided by U.S. Pat. No. 6,359,563 to Herzer. The '563 marker employs multiple strips of magnetostrictive amorphous ribbon that are cut to the same length and given the same annealing treatment. A marker having such strips disposed in registration is disclosed to produce a resonant signal amplitude that is comparable to that produced by a conventional magnetoelastic marker employing a single piece of material having about twice the width. On the other hand, a single strip of thicker ribbon, even after annealing, is disclosed not to provide a commensurate increase in resonant signal amplitude.
The '563 patent further discloses that prior art ribbon optimized for a multiple resonator tag is unsuitable for a single resonator marker and vice versa. Moreover, each of the multiple strip markers disclosed by the '563 reference employs an annealed ribbon, and not as-cast, unannealed material. A feedback controlled annealing system is said to provide extremely consistent and reproducible properties in the annealed ribbon, which otherwise is said to be subject to relatively strong fluctuations in the required magnetic properties.
There exists a need in the market place for an Accousto Magnetic label that is compatible with standard 58 Khz EAS systems; but does not deactivate or deaden during purchase of merchandise with which the label is associated. Currently retailers are using a “hard tag” that is attached to an article appointed for protection. The label is detached at the register. Contained within the “hard tag” is a ferrite adapted to trigger an alarm of an EAS system when an article is improperly taken out of the store. These deactivatable, ferrite containing tags are expensive. Application of non-deactivatable Accousto Magnetic “hard tags” to merchandise for which protection is sought would eliminate use of ferrites and save considerable costs.
There remains a need in the art for a non-deactivatable, mechanically resonant EAS marker that is inexpensive to produce, and highly reliable in operation. Also needed is a method and apparatus that produces non-deactivatable, mechanically resonant EAS markers with such precision that signals repeatedly generated by the markers in the presence of an applied magnetic field have substantially the same identifying characteristics.